1. Field of the Invention
The invention relates to a method and to a base station for forming beams for the transmit direction, in particular in radio communications systems with CDMA transmission.
2. Description of the Related Art
In radio communications systems, telecommunications (for example voice, image information or other data) are transmitted as signals using electromagnetic waves over a radio interface. The radio interface refers to a link between a base station and subscriber stations, and the subscriber stations can be mobile stations or fixed radio stations. The electromagnetic waves are emitted here with carrier frequencies which lie in the frequency band provided for the respective system. For future radio communications systems, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd generation systems, frequencies in the frequency band of approximately 2000 MHz are provided.
In communications systems, for example mobile radio networks or cordless local loop systems, methods of spatial subscriber separation are known, for example from DE 197 13 666. In such SDMA (Space Division Multiple Access) systems, a plurality of communications links can be supplied in one common channel, the channel being described by a frequency band and possibly a time slot or a spread code in SDMA/TDMA (time division multiple access) or SDMA/CDMA (code division multiple access) systems. For this purpose, adaptive antenna devices with a plurality of antenna elements are used at the transmit end, for example in base stations of mobile radio networks. By these adaptive antennas it is possible to form, by beam forming, a plurality of radiation lobes which are matched to the respective positions of the receiving radio station, for example mobile stations of mobile radio networks. By these radiation lobes which are independent of one another, the spatial resolution, which is used to separate subscriber signals, is achieved.
The transmission path from a base station to a mobile station is referred to as a downlink, and the transmission path from a mobile station of a base station is referred to as an uplink. The spatial subscriber separation by beam forming is generally used for the downlink.
The spectral efficiency of the downlink will play a significant role for future radio communications systems because a larger number of services have an asymmetrical traffic volume in favor of the downlink. Transmission methods with adaptive antennas can increase the spectral efficiency, because, for example in DS-CDMA (direct sequence CDMA), it is possible to reduce the influence of the near/far effect. In known methods for beam forming, see C. Brunner, M. Joham, W. Utschick, M. Haardt, J. A. Nossek, “Downlink Beam-forming for WCDMA based on Uplink Channel Parameters”, Proc. Third European Personal Mobile Communications Conference, 1999, it has been assumed that the transmit signal of the base station has constant properties over a certain time period—one time slot. The optimization of the beam-forming relates here to a maximization of the signal-to-noise ratio for a link.